An optical film is used as a retardation film or a protective film for a polarizing plate. The retardation film is used in a liquid crystal display, and has functions of color compensation, broadening of a viewing angle and prevention of reflection. As a retardation film, a λ/4 film and a λ/2 film are known, and a polycarbonate, a polyether sulfone and a polysulfone are used as materials therefor. λ/4 films and λ/2 films formed from these materials have the property of having a larger phase difference as the wavelength gets shorter. There is hence involved a problem that the wavelength capable of functioning as a λ/4 film and a λ/2 film is limited to specific wavelengths.
As a method of controlling a wavelength in a broad band, there is known a method of stacking two or more birefringence films having different dependency of phase difference on wavelength at specific angles (Patent Document 1). This method requires the steps of attaching a plurality of retardation films and adjusting attaching angles and hence has a problem on productivity. Further, since the thickness or the entire retardation films is increased, there is involved a problem that the light transmittance is decreased to get dark.
In recent years, there is proposed a method of controlling a wavelength in a broad band by means of one film without stacking (Patent Document 2). That is a method using a copolycarbonate composed of a unit having positive refractivity anisotropy and a unit having negative refractivity anisotropy. However, this copolycarbonate has a high melting temperature since it contains a unit derived from fluorene-based bisphenol, and there is involved a problem that a gel substance formed by decomposition is liable to be generated when it melted. Further, it has a high glass transition temperature (Tg) and requires a high temperature for film stretching, and it requires an unconventional special processing apparatus. Further, it has a high photoelastic constant and has a large birefringence due to a stress, and it has a problem that a light omission takes place when it is used as a retardation film.
On the other hand, there is already proposed a low-photoelastic-constant copolycarbonate formed from an aliphatic diol to be used for an optical disc (Patent Document 3). In this document, however, nothing has been studied with regard to the stretchability of the film and the chromatic dispersion. Further, the photoelastic constant of the copolycarbonate described in this document is required to be further decreased when it is used as a retardation film or a protective film for a polarizing plate.
Further, there is reported a low-photoelastic-constant retardation film formed from a copolycarbonate containing a fluorene component and an isosorbide component (Patent Document 4). Since this copolycarbonate is a term polymer, it is required to precisely control the compositional ratio of three components for controlling the chromatic dispersion, and its stable production is not easy. Further, since it has low thermal stability, it has a defect that its molecular weight is liable to be decreased during melt-processing.
There is also proposed a retardation film formed from a copolycarbonate containing a fluorene-based bisphenol structure (Patent Documents 5 and 6). Further, there is also proposed a polarizing plate protective film formed from a copolycarbonate containing a fluorene-based bisphenol structure (Patent Document 7). However, any one of these has a high glass transition temperature (Tg), and a high temperature is required for stretching a film, so that art unconventional special processing apparatus is required. Further, they have high photoelastic constants and large birefringence due to a stress, and they have a problem that a light omission takes place when they are used as retardation films.    (Patent Document 1) JP-A 2-120804    (Patent Document 2) Japanese Patent No. 3325560    (Patent Document 3) JP-A 2004-67990    (Patent Document 4) International Publication No. 06/041190    (Patent Document 5) International Publication No. 01/009649    (Patent Document 6) JP-A 2006-323254    (Patent Document 7) Japanese Patent No. 3995307